Recently, transmission schemes for higher transmission rates in wireless communication systems are being researched and standardized. In order to have such a high transmission rate in wireless LAN systems as well, a structure having a transmission rate of a maximum of 600 Mbps has been standardized, to which a MIMO system having multiple input/output in IEEE 802.11 TGn has been applied. There has been discussion in IEEE 802.11 VHTSG regarding a system having a maximum transmission rate of 1 Gbps at MAC SAP, and the task group of IEEE 802.11 TGac/TGad has been established accordingly. In order to maintain frequency efficiency while satisfying such a high transmission rate, the AP and STA must support more streams than four, which are supported by TGn, requiring a large number of antennas. In STA's terms, it is difficult to support a large number of antennas, considering the complexity or power consumption of the STA. Therefore, multi-user MIMO is being considered, according to which the AP simultaneously transmits to multiple STAs.
FIG. 1 is a timing diagram for explaining the occurrence of interference between stations (STAs) which simultaneously transmit data in a case in which a transmission scheme such as TGn is maintained while supporting a multi-user MIMO.
As illustrated in FIG. 1, when different data are simultaneously transmitted to two or more STAs, different information transmitted to the respective STAs may be interfered in areas indicated by reference numeral 101.
In addition, the respective STAS has different signal to interference plus noise ratios (SINRs), depending on channel states or interference degrees of the STAs. However, in a currently considered frame structure, the number of LTFs is determined by the number of streams, and a MCS is determined by a minimum transfer rate of a signal field (SIG).
In the IEEE 802.11n, a mixed PPDU format provides a backward compatibility with the IEEE. 802.11a/g, and a green field format supports only the IEEE 802.11n. Each STA sets Network Allocation Vector ((NAV):(TXOP)) information by using length information and a modulation & coding scheme contained in a signal field of a frame.
However, in a case in which a multi-user MIMO is applied, each STA receives a beamformed frame, and thus, STAs may not correctly detect length information and MCS of the signal field. Consequently, a hidden node problem may become more serious.